Ocean warming has caused coral mass bleaching and mortality worldwide and the persistence of symbiotic reef-building corals requires rapid acclimation or adaptation. Experimental evolution of the corals microalgal symbionts followed by their introduction into coral is one potential method to enhance coral thermotolerance. In this study, heat evolved microalgal symbionts of the generalist species, Cladocopium proliferum were exposed to elevated temperature 31 C for ten years, and were introduced into chemically bleached adult fragments of the scleractinian coral, Galaxea fascicularis. The new symbionts persisted in coral fragments for five months and various physiological parameters such as growth, cell density and photochemical efficiency were measured and their metabolite profiles were determined. Our findings show that adult coral thermotolerance can be enhanced via the uptake of exogenously supplied, heat evolved symbionts, without a trade-off against growth under ambient temperature.